Method and structure to enhance temperature/humidity/bias performance of semiconductor devices by surface modification

ABSTRACT

A method is disclosed of repairing wirebond damage on semiconductor chips such as high speed semiconductor microprocessors, application specific integrated circuits (ASICs), and other high speed integrated circuit devices, particularly devices using low-k dielectric materials. The method involves surface modification using reactive liquids. In a preferred embodiment, the method comprises applying a silicon-containing liquid reagent precursor such as TEOS to the surface of the chip and allowing the liquid reagent to react with moisture to form a solid dielectric plug or film ( 50 ) to produce a barrier against moisture ingress, thereby enhancing the temperature/humidity/bias (THB) performance of such semiconductor devices.

This application is a divisional of U.S. patent application Ser. No.10/906,189, attorney docket number FIS920020045US2, filed on Feb. 30,2005, which is a national phase application of International ApplicationNo. PCT/US2002035116, filed on Oct. 31, 2002.

TECHNICAL FIELD

This invention relates generally to the manufacture of high speedsemiconductor microprocessors, application specific integrated circuits(ASICs), and other high speed integrated circuit devices. Moreparticularly, this invention relates to a method to enhance thetemperature/humidity/bias (THB) performance of such semiconductordevices, particularly devices using low-k dielectric materials, bysurface modification using reactive liquids.

BACKGROUND OF THE INVENTION

Metal interconnections in very large scale integrated (VLSI) orultra-large scale integrated (ULSI) circuits typically consist ofinterconnect structures containing patterned layers of metal wiring.Typical integrated circuit (IC) devices contain from three to fifteenlayers of metal wiring. As feature size decreases and device areadensity increases, the number of interconnect layers is expected toincrease.

The materials and layout of these interconnect structures are preferablychosen to minimize signal propagation delays, hence maximizing theoverall circuit speed. An indication of signal propagation delay withinthe interconnect structure is the RC time constant for each metal wiringlayer, where R is the resistance of the wiring and C is the effectivecapacitance between a selected signal line (i.e., conductor) and thesurrounding conductors in the multilevel interconnect structure. The RCtime constant may be reduced by lowering the resistance of the wiringmaterial. Copper is therefore a preferred material for IC interconnectsbecause of its relatively low resistance. The RC time constant may alsobe reduced by using dielectric materials with a lower dielectricconstant, k.

Many performance advantages are obtained by using a low-k dielectricmaterial as the inter-level dielectric (ILD) for back-end-of-line (BEOL)interconnects of high speed microprocessors, application specificintegrated circuits (ASICs) and related integrated circuit devices. Inadvanced interconnect structures, the ILD is preferably a low-kpolymeric thermoset material such as SiLK™ (an aromatic hydrocarbonthermosetting polymer available from The Dow Chemical Company). Otherpreferable low-k dielectric materials include carbon-doped silicondioxide (also known as silicon oxycarbide or SiCOH dielectrics);fluorine-doped silicon oxide (also known as fluorosilicate glass, orFSG); spin-on glasses; silsesquioxanes, including hydrogensilsesquioxane (HSQ), methyl silsesquioxane (MSQ) and mixtures orcopolymers of HSQ and MSQ; and any silicon-containing low-k dielectric.

There are several fabrication advantages when an organic thermosetpolymer is selected. The primary advantages of organic thermoset polymerdielectrics are lower dielectric constant (typically about 2.65), lowercracking rate under applied stress, and etch (RIE) selectivity. Glassdielectric materials such as SiCOH or carbon-doped oxide tend to crackunder applied stress and in humid atmosphere, while organic thermosetpolymers do not. Moreover, the carbon-based thermoset polymers are notetched in the RIE chemistry used to open the cap at the bottom of eachvia, while silicon-based SiCOH is etched in this etch step. In otherwords, carbon-based thermoset polymers exhibit high etch selectivitywhile silicon-based SiCOH exhibits low selectivity. Finally, organicthermoset polymer materials are spin applied, whereas glasses aretypically applied using plasma-enhanced chemical vapor deposition (PECVD) tools. Spin apply tools have lower cost of ownership than PE CVDtools.

However, one disadvantage is the low modulus of organic polymer or low kCVD dielectrics, which can result in defects or breaches formed in thepassivation layers when the completed IC chip is electrically connectedvia wire bonding (or soldering) methods to an IC holder. For example,cracks may form in the passivation layers in the vicinity of bond pads.Such cracks typically have a width of 1000 angstroms to 5000 angstroms,a depth of several microns, and a length of 1 micron to 100 microns.Cracks often occur as well at the terminal insulator level of the chip.These cracks have many causes; most notable are those caused by roughhandling of the finished chip prior to packaging and the damage that canresult during the die wire bonding process. Delamination of thepassivation layers is also a possible problem.

Although such microcracks may have been present in prior devices usingconventional inorganic dielectric materials, the nature of theintegration in those devices minimized the deleterious impact of suchdefects on product reliability. With the advent of low-k dielectrics andtheir inherent inferior mechanical properties with respect to inorganicoxides, an increase in sensitivity and amount of microcracking at theterminal insulator level of the chip has been observed.

In devices comprising low-k dielectric materials, the terminal insulatorlevel of the chip is often built utilizing an inorganic oxide layer as amoisture barrier. However, this terminal inorganic oxide is more easilydamaged than in devices using oxide for all inter-layer dielectric, dueto the unique mechanical structure of low-k dielectric integration.Furthermore, as device ground rules continue to decrease, even withoutthe transition to low-k dielectrics, these microcracks and delaminationsare likely to become a significant source of device performancedegradation if they are left unrepaired before final packaging andencapsulation.

Post wire-bond packaging and encapsulation processes are ineffective insealing these cracks in the terminal insulator level of the chip. As aresult of the lack of seal or repair of these cracks in the terminalinsulator level of the chip, electrical degradation in semiconductorchip electrical performance has been observed during temperature andhumidity stress testing. For example, U.S. Pat. No. 5,689,089 disclosesthe use of silicone-based polymers for encapsulation. However, it hasbeen observed that such polymers alone can not provide an effectivebarrier to moisture ingress.

More complex schemes using new materials in the passivation layers andin the metal bond pads have been proposed. For example, the WaferApplied Seal for PEM Protection (WASPP), sponsored by The U.S. ArmyManufacturing Technology (ManTech) Program, is a high cost multi-layeredapproach. A spin applied material (such as hydrogen silsesquioxane, HSQ)and a PE CVD applied material (silicon carbide) are used forpassivation. Also, two metal layers (gold plus titanium) are added tothe bond pad. However, the use of unconventional materials andadditional metal layers adds significantly to the cost of themanufacturing process.

Thus, there remains a need in the art for a low-cost method to enhancetemperature/humidity/bias performance of semiconductor devices.

DISCLOSURE OF THE INVENTION

It is therefore an object the present invention to provide a method forrepairing or sealing damage on semiconductor chips due to electricalconnections such as wire bonds, and more generally for enhancingtemperature/humidity/bias performance of semiconductor chips using alow-cost method which does not introduce unconventional materials oradditional metal layers.

In one aspect, the present invention is directed to an integratedcircuit chip comprising a first dielectric material at the top surfaceof the integrated circuit chip; electrical connections attached to thetop surface of the integrated circuit chip; and a layer of a seconddielectric material atop the top surface of the integrated circuit chip,wherein the second dielectric material is a silicon-containing conformalencapsulant.

In another aspect, the present invention is directed to a method forencapsulating an integrated circuit chip, the method comprising thesteps of: attaching electrical connections to a top surface of anintegrated circuit chip, wherein the top surface comprises a firstdielectric material; depositing a second material atop the top surfaceof the integrated circuit chip, wherein the second material comprises asilicon-containing conformal encapsulant precursor; and treating thedeposited second material to cause reaction of the precursor to form asecond dielectric material.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The drawings are for illustration purposes only and arenot drawn to scale. Furthermore, like numbers represent like features inthe drawings. The invention itself, however, both as to organization andmethod of operation, may best be understood by reference to the detaileddescription which follows, taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1A and 1B illustrate plan views of bond pads on a semiconductorchip, with a wirebond connected to the bond pad and microcracks adjacentto the wirebond;

FIGS. 2A and 2B illustrate cross-sectional views of bond pads on asemiconductor chip, with a wirebond connected to the bond pad andmicrocracks adjacent to the wirebond; and

FIGS. 3A and 3B illustrate cross-sectional views of the bond pads ofFIGS. 2A and 2B, with the surface modification of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described by reference to the accompanyingfigures. In the figures, various aspects of the structures have beenshown and schematically represented in a simplified manner to moreclearly describe and illustrate the invention. For example, the figuresare not intended to be to scale. In addition, the verticalcross-sections of the various aspects of the structures are illustratedas being rectangular in shape. Those skilled in the art will appreciate,however, that with practical structures these aspects will most likelyincorporate more tapered features. Moreover, the invention is notlimited to constructions of any particular shape.

In one aspect, this invention is a method to repair and seal microcracksand delaminations that are found in typical damage due to wire bonds orother electrical connections. A damaged part may be, for example, anintegrated circuit chip mounted on a chip carrier or interposer, inwhich the chip contains damage which includes microcracks, fissures,delaminations and the like within the passivation layers of the chip.

FIGS. 1A and 1B illustrate plan views of a typical bond pad on asemiconductor chip, with a wire bond attached to the bond pad. Bond pad10 is typically formed of aluminum, but may be formed of any suitableconductive material. Dielectric material 30 surrounds bond pad 10. Wirebond 20 is attached to bond pad 10 at the end 22 thereof. In FIGS. 1Aand 1B, end 22 of wire bond 20 is shown attached to bond pad 10off-center. When the wire bond is attached to the bond pad off-center,microcracks 40 may occur in the aluminum bond pad 10 and surroundingdielectric material 30. Microcracks having a length of about 20 micronshave been observed.

FIGS. 2A and 2B illustrate cross-sectional views of a typical bond padon a semiconductor chip, with a wire bond attached to the bond pad. Bondpad 10, typically of aluminum, is formed over conductor 12, typically ofcopper. Conductor 12 is surrounded by dielectric material 31, which maybe any suitable dielectric material, such as a polymeric low-kdielectric material. Dielectric 31 is typically capped with one or moredielectric layers, such as silicon dioxide (SiO₂) layer 32 and siliconnitride (SiN) layer 33. In FIG. 2A, bond pad 10 is shown formed overcapping layers 32 and 33, and two additional capping layers 34, 35 aredeposited over bond pad 10 and capping layer 33. Capping layers 34 and35 may be the same as or different from capping layers 32 and 33. Forexample, capping layer 34 may be SiO₂ and capping layer 35 may be SiN.Above the final capping layer, a layer of protective material 36 may bedeposited, such as a photosensitive polyimide material. Wire bond 20 isshown attached to bond pad 10 at its end 22. Microcracks 40 are alsoshown extending through the aluminum bond into the capping layers. Notethat such microcracks may occur even when wire bond 20 is centered onbond pad 10. Delaminations (not shown) may also occur between or beneathany of capping layers 32, 33, 34 and 35.

FIG. 2B shows an alternative bond pad configuration, wherein bond pad 10is formed over capping layers 32-35, and protective material 36 isdeposited over the vertical sides of bond pad 10. Even with suchpolyimide protective material 36 covering the aluminum bond pad 10,microcracks 40 have been observed extending through protective material36, bond pad 10 and into the underlying dielectric materials 31-35.Again, delaminations (not shown) may also occur between or beneath anyof capping layers 32-35.

Such microcracks may be repaired or sealed using the method of thisinvention, which comprises applying a reactive liquid reagent to thedamaged part, after bonding operations and before the post bondpackaging and encapsulation process. FIGS. 3A and 3B illustrate the bondpad configurations of FIGS. 2A and 2B, with a layer of reactive liquidreagent 50 applied over the damaged semiconductor chip. The liquidreagent fills and wets the damaged areas on the chip, such asmicrocracks 40.

The liquid reagent may comprise one or more of the following reactivematerials: alkoxy silanes, siloxanes, silazanes, epoxy-siloxanes(including alkoxy-epoxy-siloxanes), epoxy-silazanes, urea-silazanes,carbo-silazanes, urea-siloxanes, carbo-siloxanes, and other similarlyreactive materials, including such compounds as TEOS (tetraethoxysilane, tetraorthosilicate), HMDS (hexamethyldisilazane), TMCTS(tetra methyl cyclo tetra siloxane), and 3MC3S (tri methyl cyclo trisiloxane). Alternatively, the liquid reagent may comprise an inorganicpolymer including polysilazanes, polyureasilazanes, polycarbosilazanes,polysiloxanes, polyureasiloxanes, polycarbosiloxanes, and relatedsilicon-containing polymers. These polymers may be of low molecularweight and/or may be diluted with solvent to reduce the viscosity.

It is preferred that, upon exposure to moisture, the liquid reagentreacts with water, forming a solid dielectric material. For example,TEOS may be applied to the damaged part to seal microcracks at theterminal insulator level of the chip. The TEOS reacts with moisture toform a TEOS oxide barrier against moisture ingress. It is particularlypreferred that the liquid reagent reacts with moisture contaminationalready present in the damaged area to dehydrate or bond this moistureso that it is not detrimental to the performance of the semiconductorchip as an electrical device.

Alternatively, the liquid reagents may react with a secondary initiatorto bond with the semiconductor substrate and themselves to seal damagedareas from moisture ingress. Typical secondary initiators are dependenton the type of binary system used. For moisture-activated systems,organic acids or alcohols may enhance the chemical reaction. Typicalorganic acids include acetic, formic and propionic acid. Typicalalcohols include methyl, ethyl and isopropyl. The solution of water inan alcohol, such as 25% w/w water in methanol, may improve theperformance of an alcohol or water alone as the alcohol lowers theviscosity and improves the wetting action of water while the siloxanesand silazanes are generally more reactive towards water than alcohols.Furthermore, lower molecular weight organic initiators are preferred asthey are generally more reactive and have a lower viscosity.

For binary systems such as epoxy-silanes, epoxy-siloxanes andepoxy-silazanes, an oxidizer may be used, such as a solution of aperoxide (such as benzyl peroxide) in xylene or hexane. Alternatively,peroxide solutions in ethers such as diethyl ether or tetrahydrofuran(THF) may be used as the binary initiator. Peroxide solutions may alsobe used with moisture-activated binary systems. Since these peroxidesolutions may be unstable, it is preferred that they be mixed in smallvolumes just before use.

While higher molecular weight hydrocarbon solvents such as mineral oilwould be more stable as a solvent for peroxide solution, the higherviscosity of this solvent is a major disadvantage. Some mineral oil maybe added to hexane or xylene solutions of organic peroxides such asbenzyl peroxide to lower flashpoint, but lower viscosity solutions aremost preferred.

Other oxidative initiators such as ozone are not to be precluded and insome cases ozone initiation may be preferred, such as when themetallurgy and package are resistant to degradation by ozone.

The liquid reagent should be a reactive, low viscosity,silicon-containing liquid precursor. Materials from the following groupare effective in producing a seal and repair of microcracks at theterminal insulator level of the chip: alkoxy silanes, siloxanes,silazanes, epoxy-siloxanes (including alkoxy-epoxy-siloxanes),epoxy-silazanes, urea-silazanes, carbo-silazanes, urea-siloxanes,carbo-siloxanes, TEOS (tetra ethoxysilane, tetraorthosilicate), HMDS(hexamethyldisilazane), TMCTS (tetra methyl cyclo tetra siloxane), 3MC3S(tri methyl cyclo tri siloxane), polysilazanes, polyureasilazanes,polycarbosilazanes, polysiloxanes, polyureasiloxanes,polycarbosiloxanes, related silicon-containing polymers, and othersimilar materials.

The liquid reagent should have a low viscosity such that the fluid willenter and fill fine cracks or crevices, and reagent should also wet wellto the terminal insulator structure of conductor and dielectric to avoidbridging or non-coating of the defective areas that are to be repaired.The liquid reagent should be able to form a chemical bond with thesestructures, and should be able to react to form a plug or film thatproduces a barrier against moisture ingress.

The liquid reagent, such as alkoxy silane or other reactive siloxanes orsilazanes, should be applied to the chip with sufficient volume to fillall damaged areas. The liquid reagent may be applied only to damagedareas of the chip, or may be applied across the entire surface of thechip as shown in FIGS. 3A and 3B to encapsulate the semiconductorsubstrate, providing an extra barrier layer to resist moisture ingress.

Reactive reagents such as the reagents described above are particularlypreferred because they produce no undesirable by-products, such asstrong mineral acids or corrosive salts, during the reaction and cure ofthese materials. The reagents described above are also preferred becausethese materials are insulators or dielectric materials and therefore donot degrade the performance of the chip package interconnection.

While FIGS. 1 and 2 illustrate wire bond connections to the chip, solderball arrays are commonly used in the industry to form high speedinput/output connections to a completed chip.

The solder ball array may connect the chip to a chip carrier,interposer, or ceramic package. In an alternative embodiment of thisinvention, microcracks or delamination or similar damage within thepassivation layers may be present after the operations of dicing, waferlevel probing, or post reflow and chip attachment. In this embodiment,the liquid reagent is then applied to the chip mounted on the chipcarrier, interposer, or ceramic package after the reflow and chipattachment operation. The secondary initiator may then be applied.

In a preferred embodiment, the method of repairing and sealing thedamaged chip comprises subjecting the semiconductor chip to a vacuum,for example a vacuum of about 10×10⁻³ torr, applying the liquid reagentor sealant to the chip, and subjecting the chip to a thermal and/orultraviolet (UV) light cure. It is preferred that the chip is subjectedto thermal cure, and the preferred temperature range is about 20 degreesC. below the lowest solder (braze) liquids in the package. A mostpreferred temperature range is about 150 to 220 degrees C. A cure timeof about 1 minute to 1 hour is preferred, with about 5 minutes to 30minutes most preferred.

Alternatively, the chip may be subjected to UV cure. A preferred UVlight source is a high pressure Hg or Xe/Hg lamp. It is most preferredthat UV cure is used in conjunction with a thermal treatment at atemperature in the range of about 150 to 220 degrees C. A cure time ofabout 5 minutes to 30 minutes is most preferred.

In another embodiment, the method comprises subjecting the semiconductorchip to a vacuum of about 10×10⁻³ torr, applying the liquid reagent orsealant to the chip, increasing the pressure up to about 5 atmospheres,and subjecting the chip to a thermal and/or UV light cure.

In yet another embodiment, the method comprises applying the liquidreagent or sealant at ambient pressure, and subjecting the chip to athermal and/or UV light cure.

In yet another embodiment, the method comprises applying the liquidreagent or sealant at ambient pressure, increasing the pressure up toabout 5 atmospheres, and subjecting the chip to a thermal and/or UVlight cure.

In yet another embodiment, the method comprises subjecting thesemiconductor chip to a vacuum of about 10×10⁻³ torr, applying theliquid reagent or sealant to the chip, and applying a secondaryinitiator or chemical curing agent.

In yet another embodiment, the method comprises subjecting thesemiconductor chip to a vacuum of about 10×10⁻³ torr, applying theliquid reagent or sealant to the chip, increasing the pressure up toabout 5 atmospheres, and then applying a secondary initiator or chemicalcuring agent.

In yet another embodiment, the method comprises applying the liquidreagent or sealant at ambient pressure, and applying a secondaryinitiator or chemical curing agent.

In yet another embodiment, the method comprises applying the liquidreagent or sealant at ambient pressure, increasing the pressure up toabout 5 atmospheres, and applying a secondary initiator or chemicalcuring agent.

While the present invention has been particularly described inconjunction with a specific preferred embodiment and other alternativeembodiments, it is evident that numerous alternatives, modifications andvariations will be apparent to those skilled in the art in light of theforegoing description. It is therefore intended that the appended claimsembrace all such alternatives, modifications and variations as fallingwithin the true scope and spirit of the present invention.

1. An integrated circuit chip comprising: a first dielectric material(35) at the top surface of said integrated circuit chip; electricalconnections (20) attached to the top surface of said integrated circuitchip; and a layer of a second dielectric material (50) atop the topsurface of said integrated circuit chip, wherein said second dielectricmaterial is a silicon-containing conformal encapsulant.
 2. Theintegrated circuit chip of claim 1, wherein said electrical connections(20) are wire bonds.
 3. The integrated circuit chip of claim 1, whereinsaid electrical connections (20) are soldered connections attaching thechip to a chip carrier, interposer, or ceramic package.
 4. Theintegrated circuit chip of claim 1, wherein said first dielectricmaterial (35) at the top surface contains microcracks (40), and saidsecond dielectric material (50) conformally fills the microcracks (40).5. The integrated circuit chip of claim 4, wherein the microcracks (40)have a width of 1 micron or less, and a depth of 10 microns or less. 6.The integrated circuit chip of claim 4, wherein the microcracks (40)have a width of about 0.5 micron, and a depth of about 1 to about 5microns.
 7. The integrated circuit chip of claim 1, wherein theintegrated circuit chip contains a delamination beneath said firstdielectric material (35), and said second dielectric material (50)conformally fills the delamination.
 8. The integrated circuit chip ofclaim 1, wherein the second dielectric material (50) comprises areactive, low viscosity, silicon-containing material selected from thegroup consisting of alkoxy silanes, siloxanes, silazanes,epoxy-siloxanes (including alkoxy-epoxy-siloxanes), epoxy-silazanes,urea-silazanes, carbo-silazanes, urea-siloxanes, carbo-siloxanes,polysilazanes, polyureasilazanes, polycarbosilazanes, polysiloxanes,polyureasiloxanes, polycarbosiloxanes, related silicon-containingpolymers, TEOS (tetra ethoxysilane tetraorthosilicate), HMDS(hexamethyldisilazane), TMCTS (tetra methyl cyclo tetra siloxane) and3MC3S (tri methyl cyclo tri siloxane).